Digital cellular telephone systems use, for instance, data transmission based on time division multiple access in the communication between a base station and a mobile station. The time division GSM system uses 8 time slots, which makes it possible for 8 mobile stations to communicate simultaneously with the base station on the same transmit/receive frequency. Each mobile station reserves one time slot from the frame and the speech or data signal to be transmitted is packed in this time slot in a coded form. The frame timing of mobile phones in the GSM system is not very time-critical to be controlled by a signal processor because of a fairly extensive length of the time slot (577 .mu.s) and a low bit rate (270 kbit/s). Furthermore, simultaneous transmission and reception is not required of GSM mobile stations.
As systems based on the TDMA develop, the requirements for mobile phone timing increase because higher data transmission capacity is required, and as a consequence, traffic in different types of cells is required. Cell types differ from each other with respect to bit rates and burst lengths. The burst length of a macro cell is higher and the bit rate lower than those of a micro or pico cell. In this case the requirement for a fast handover from one cell type to another without a break in the data transmission becomes essential.
Especially when transferring data at high bit rate, the mobile phone has to receive a short burst with a high bit rate, while simultaneously transmitting a longer burst with a lower bit rate.
Instead, the clock frequency in a micro cell of the TDMA system is higher, and especially when the mobile station moves between different cell types, such as macro cells and micro/pico cells, higher bit rate and accuracy in the frame timing is required.